json - Haskell Aeson：如何将 Value 转换为自定义类型？

Question

找不到好的例子。感谢任何帮助。JSON如下：

[{
  "EXIF:Make": "Canon",
  "EXIF:Model": "Canon PowerShot S95",
  "EXIF:Orientation": "Horizontal (normal)",
  "EXIF:XResolution": 180,
  "EXIF:YResolution": 180,
  "EXIF:ResolutionUnit": "inches"
}]

我使用的代码如下：

import Data.Aeson
import Data.Attoparsec
import Data.ByteString
x <- fmap (parse json) (Data.ByteString.readFile "json.txt")

如何定义和使用FromJSON要转换x为的类型：

data Exif = Exif [[(String, String)]]

或类似的数据结构？请注意[[]]——我希望 JSON 有多个顶级条目。

Answer 1

这是一个惯用的解决方案：

{-# LANGUAGE OverloadedStrings #-}

module Main
       where

import Control.Applicative
import Control.Monad
import Data.Aeson
import Data.Attoparsec

import qualified Data.ByteString as B
import qualified Data.Text as T

data ExifEntry = ExifEntry { exifMake :: T.Text,
                             exifModel :: T.Text,
                             exifOrientation :: T.Text,
                             exifXResolution :: Int,
                             exifYResolution :: Int,
                             exifResolutionUnit :: T.Text
                           } deriving (Eq, Show)


instance FromJSON ExifEntry
  where
    parseJSON (Object v) = ExifEntry <$> 
                           v .: "EXIF:Make" <*>
                           v .: "EXIF:Model" <*>
                           v .: "EXIF:Orientation" <*>
                           v .: "EXIF:XResolution" <*>
                           v .: "EXIF:YResolution" <*>
                           v .: "EXIF:ResolutionUnit"
    parseJSON _          = mzero


parseAll :: B.ByteString -> [ExifEntry]
parseAll s = case (parse (fromJSON <$> json) s) of
  Done _ (Error err)  -> error err
  Done ss (Success e) -> e:(parseAll ss)
  _                   -> []

main :: IO ()
main = do s <- B.readFile "json.txt"
          let p = parseAll s
          print p

测试：

$ cat json.txt 
{
  "EXIF:Make": "Canon",
  "EXIF:Model": "Canon PowerShot S95",
  "EXIF:Orientation": "Horizontal (normal)",
  "EXIF:XResolution": 180,
  "EXIF:YResolution": 180,
  "EXIF:ResolutionUnit": "inches"
}

{
  "EXIF:Make": "Canon",
  "EXIF:Model": "Canon PowerShot S995",
  "EXIF:Orientation": "Horizontal (normal)",
  "EXIF:XResolution": 180,
  "EXIF:YResolution": 180,
  "EXIF:ResolutionUnit": "inches"
}
$ ./dist/build/test/test
[ExifEntry {exifMake = "Canon", exifModel = "Canon PowerShot S95", exifOrientation = "Horizontal (normal)", exifXResolution = 180, exifYResolution = 180, exifResolutionUnit = "inches"},ExifEntry {exifMake = "Canon", exifModel = "Canon PowerShot S995", exifOrientation = "Horizontal (normal)", exifXResolution = 180, exifYResolution = 180, exifResolutionUnit = "inches"}]

或者，这是一个稍微丑陋的解决方案，它为您提供您请求的数据类型 ( [[(Text,Text)]])。